The European ACEA engine oil specifications require passing a lubricant engine test called the PEUGEOT TU5. The PEUGEOT TU5 test is based on a 1.6 liter, 4 cylinder gasoline engine to which a fixed charge of engine oil has been added. The engine is run for 72 hours at full load conditions with the oil temperature held at 150° C. No oil top up is allowed during the test. The increase in engine oil viscosity over the 72 hour period is measured and must be below proscribed limits in order for the test to comply with the ACEA specifications. The viscosity limits are set in terms of Absolute Viscosity Increase (measured in mm2/sec) relative to the viscosity increase of reference oil.
Although some oil chemical oxidation of the oil may take place during the test, the primary mechanism for viscosity increase using the PEUGEOT TU5 test is believed to be a physical one. Contact between the hot blow-by gases and the lubricant may cause heat induced evaporation of light base oil & concentration of heavy oil components in the oil sump oil. The use of chemical additives such as antioxidants is generally not effective to control what is believed to be a physical mechanism for viscosity increase.
Relatively high shear stability index (SSI) olefin copolymer (OCP) viscosity index improvers (VII's) may help limit the viscosity increase experienced during the PEUGEOT TU5 test, however, in order to be effective, the additive package may require that a majority of the additive package be the OCP VII. Such an amount of high SSI OCP VII's in the additive package may be detrimental to providing a lubricant that can also pass a BOSCH 30 cycle shear test which is also found in the ACEA engine oil specifications. High SSI OCP VII's are typically not compatible with other additive package components and may have to be blended into the oil separately from the additive concentrate. Another disadvantage of using a significant amount of OCP VII is that the OCP component may make it difficult to meet the 3.5 minimum HTHS requirements of 10W30 ACEA A3/B3 oils.
Other viscosity index improvers that may be useful for limiting the viscosity increase experienced during the PEUGEOT TU5 test are tri-block copolymers of hydrogenated isoprene-styrene. However, such tri-block copolymers are a relatively expensive component with limited availability. Accordingly, in order to pass the ACEA European oil sequence test, relatively low SSI OCP VII's must be used with larger amounts of more expensive base oils such as hydro-cracked Group III and/or polyalphaolefin (PAO) Group IV base oils having relatively lower Noack volatilities.
With regard to the foregoing, embodiments of the disclosure may provide a lubricant composition, additive concentrate, and method for lubricating an engine. The crankcase lubricant has less than 30 weight percent of a relatively low Noack volatility base oil and a viscosity additive. The viscosity additive has a minor effective amount of sacrificial polymeric viscosity index improver having a shear stability index (SSI) of greater than about 45 and a major amount of a viscosity index improver having an SSI of less than about 26. The minor amount of sacrificial viscosity index improver additive is effective to provide a lubricant composition having a ratio of absolute viscosity increase at 40° C. to an absolute viscosity increase at 40° C. of an RL 216 reference oil of less than about 0.8.
In another embodiment, the disclosure provides an additive concentrate for a lubricant. The additive concentrate includes a sacrificial polymeric viscosity index improver having a shear stability index (SSI) of greater than about 45 and major amount of a viscosity index improver having an SSI of less than about 26. A fully formulated lubricant containing the viscosity additive concentrate includes a major amount of base oil having a Noack volatility ranging from about 7.0 to about 15 wt. %. The additive concentrate is effective to provide a lubricant composition having a ratio of absolute viscosity increase at 40° C. to an absolute viscosity increase at 40° C. of an RL 216 reference oil of less than about 0.8.
Yet another embodiment of the disclosure provides a method for lubricating an engine. According to the method, a lubricant composition is provided that includes a major amount of base oil having a Noack volatility ranging from about 7.0 to about 15 weight percent and a viscosity additive for the base oil. The viscosity additive has an effective amount of sacrificial polymeric viscosity index improver having a shear stability index (SSI) of greater than about 45 and a major amount of viscosity index improver having an SSI of less than about 26. The viscosity additive is effective to provide a lubricant composition having a ratio of absolute viscosity increase at 40° C. to an absolute viscosity increase at 40° C. of an RL 216 reference oil of less than about 0.8.
Exemplary embodiments of the disclosure described herein relate to the use of a relatively low treat rate of a relatively high SSI dispersant viscosity index improver additive in combination with one or more conventional relatively low SSI viscosity index improvers as a means to reduce an absolute viscosity increase of a crankcase lubricant when performing a PEUGEOT TU5 engine test. Hence the disclosure enables the use of relatively low cost, less shear stable viscosity index improvers as the bulk viscosity index improver for the lubricant composition without the need for major quantities of relatively low volatility base oils being present in the final formulation.